The circular polarization (CP) that the photoluminescence inherits from the excitation source in听nmonolayers of transition-metal dichalcogenides听(MX2)n听has been previously explained as a special feature of听odd听values of听n, where the inversion symmetry is absent. This 鈥渧alley polarization鈥� effect results from the fact that, in the absence of inversion symmetry, charge carriers in different band valleys could be selectively excited by different circular polarized light. Although several experiments observed CP in centrosymmetric听MX2听systems, e.g., for bilayer听MX2, they were dismissed as being due to some extrinsic sample irregularities. Here we show that also for听n=even, where inversion symmetry is present and valley polarization physics is strictly absent, such intrinsic selectivity in CP is to be expected on the basis of fundamental spin-orbit physics. First-principles calculations of CP predict significant polarization for听n=2听bilayers: from 69% in听MoS2听to 93% in听WS2. This realization could broaden the range of materials to be considered as CP sources.
Q. Liu, X. Zhang, A. Zunger,"" Physical Review Letters 114, 087402 (2015).(PDF)
